The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction: The reaction (1) is mildly exothermic.
Mordenite is one of a number of molecular sieve catalysts useful in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite exhibiting a network of silicon and aluminum atoms interlinked by oxygen atoms within the crystalline structure. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, 1981, under the heading “Molecular Sieves,” Vol. 15, pages 638-643, which is herein incorporated by reference. Mordenite, as found in nature or as synthesized to replicate the naturally occurring zeolite, typically exhibits a relatively low silica to alumina mole ratio of about 10 or less. Also known, however, are mordenite catalysts exhibiting a substantially lower alumina content. These aluminum deficient mordenite catalysts exhibit silica to alumina ratios greater than 10, ranging up to about 100, and may be prepared by direct synthesis as disclosed for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies. Both the typical and the aluminum deficient mordenites are known to be useful in the disproportionation of toluene.
General operating conditions relating to the disproportionation of toluene feedstock include temperatures ranging from about 200° C. to about 600° C. or above, and pressures ranging from atmospheric to perhaps 100 atmospheres or above. The specific catalyst, however, may impose constraints on reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art suggests the use of relatively high temperatures when employing the high aluminum mordenites (low silica to alumina ratios) and somewhat lower temperatures when employing low alumina mordenites. Accordingly, where mordenite catalysts exhibiting high silica to alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range.
U.S. Pat. No. 4,665,258 to Butler, however, discloses a toluene disproportionation process employing an aluminum deficient mordenite catalyst, involving a temperature range of 370° C. to 500° C. The mordenite catalysts described therein exhibit silica/alumina ratios of at least 30 and, more desirably, within the range of 40-60. The toluene weight hourly space velocity (WHSV) may be greater than 1. Hydrogen is supplied to the reaction zone at a hydrogen/toluene mole ratio within the range of 3-6, at a pressure of 500 psi or more.
Butler '258 also discloses passing a hot preflush gas, nitrogen or hydrogen, to the reaction zone prior to initiating the disproportionation reaction. The preflush gas is heated to a temperature sufficient to substantially dehydrate the catalyst by the time the toluene feed is started. This measure enables the disproportionation process to initially be performed at a somewhat lower temperature and without reduction in toluene conversion. As the disproportionation proceeds, temperature progressively increases to maintain toluene conversion at the desired level, typically about 80 percent of theoretical
U.S. Pat. No. 4,723,049 to Menard discloses toluene disproportionation carried out over aluminum deficient mordenite of the type disclosed in the aforementioned patent to Butler, with a reaction zone temperature of 370° C. to 500° C. Menard '049 employs an interruption procedure whereby the supply of toluene to the reaction zone is interrupted while the supply of hydrogen is continued. This mode of operation is disclosed to enhance the aging quality of the catalyst and show a reduction in reactor zone temperature without a corresponding decrease in toluene conversion.
It is also a common practice to promote an aluminum deficient mordenite catalyst with a catalytically active metallic content. For example, U.S. Pat. No. 3,476,821 to Brandenburg discloses disproportionation reactions employing mordenite catalysts having silica/alumina ratios within the range of 10-100 and preferably within the range of about 20-60. The mordenites are modified by the inclusion of a sulfided metal selected from the Group VIII metals. The especially preferred sulfided Group VIII metals are cobalt and nickel present in a concentration of 0.5-10 weight percent. Brandenburg '821 discloses temperature ranges from about 400° F.-750° F. The metal promoters are said to substantially increase activity and catalyst life, as indicated by runs extended over several hours or days.
As noted previously, hydrogen is commonly supplied along with toluene to the reaction zone. While the disproportionation reaction (1) does not involve chemical consumption of hydrogen, the use of hydrogen co-feed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above mentioned patent to Brandenburg '821. The amount of hydrogen supplied, which is normally measured in terms of the hydrogen/toluene mole ratio, is generally shown in the prior art to increase as temperature increases.
Bhavikatti, “Toluene Disproportionation Over Aluminum-Deficient and Metal-Loaded Mordenites. 1. Catalytic Activity and Aging”, Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 102-105, discloses toluene disproportionation at 400° C. over mordenite catalysts having silica/alumina mole ratios ranging from 12 to 61 at atmospheric pressure and a space velocity (WHSV) of 1. Bhavikatti indicates that an increase in the silica/alumina mole ratio decreases catalyst activity, while aging quality is increased (i.e. lowering aging rates). Catalyst decay was also suppressed by loading mordenites with nickel.
U.S. Pat. No. 3,562,345 to Mitsche discloses the use of molecular sieves such as mordenite catalysts in the disproportionation of toluene. The catalysts are characterized by a silica/alumina mole ratio of from about 6 to about 12, pore openings of from about 3 to about 18 angstroms and the incorporation of catalytically active metallic materials in the oxidized or reduced state, particularly Group VIB and Group VIII metals including molybdenum, tungsten, chromium, iron, nickel, cobalt, platinum, palladium, ruthenium, rhodium, osmium, and iridium. Mitsche '345 discloses transalkylation at temperatures from about 200° C. to about 480° C. and gives specific examples of transalkylation of toluene at temperatures of 420° C. to 450° C.
U.S. Pat. No. 3,677,973 to Mitsche, discloses the use of mordenite catalysts composited with an alumina salt providing a silica/alumina mole ratio of about 10 to about 30 in the disproportionation of toluene. The reaction conditions proposed in Mitsche '973 appear similar to those set forth in the aforementioned Mitsche '845 patent and, like the former patent, Mitsche '973 discloses incorporating Group VIB and Group VIII metals into the catalyst.
U.S. Pat. No. 4,151,120 to Marcilly discloses a process for the manufacture of a hydrocarbon conversion catalyst involving incorporating cobalt, nickel, silver or palladium in a mordenite catalyst having a silica/alumina mole ratio within the range of 10-100. Following incorporation of the metal into the mordenite, the catalyst is dried and subjected to a dry calcination procedure at a temperature within the range of 300° C.-700° C. in the presence of an inert or oxidizing gas having a moisture content of less than 1 percent. Marcilly '120 discloses various examples of the dismutation of toluene under reaction conditions 420° C., 30 bars, a space velocity (WHSV) of 5 and a hydrogen/hydrocarbon mole ratio of 5.
U.S. Pat. No. 4,723,048 to Dufresne discloses a process for the dismutation of toluene employing a zeolite catalyst modified by the inclusion of metals. The catalyst is described as a sodium-containing mordenite in the nature of a so-called “wide pore” mordenite, i.e., mordenite with main pores exhibiting a diameter of 7-10 Angstroms or “small pore” mordenite, mordenites with main pores exhibiting a diameter of 4-6 Angstroms. The mordenites are treated to extract sodium therefrom to provide not more than one percent by weight sodium ions and preferably not more than 0.5 percent by weight sodium ions.
Of the aforementioned references, none of them teach or suggest the disproportionation or conversion of toluene feedstock containing other heavy aromatic compounds, such as trimethylbenzenes and ethyltoluenes. In the reforming process, heavier aromatic compounds, i.e. aromatic compounds of C8 or greater, are often produced that have lesser value than other lighter aromatic compounds, such as benzene and xylene. Typically, these heavier aromatic reformates are used in gasoline blending along with toluene. Reducing the amounts of these heavier aromatics in the gasoline pool is beneficial, however, as more premium gasoline can be produced. Additionally, it is a great advantage if these heavier aromatics can be converted into more commercially valuable products, such as benzene, toluene and xylene.
U.S. Pat. No. 5,475,180 to Shamshoum, discloses the conversion of heavier aromatics during the disproportionation of toluene using a nickel-promoted mordenite catalyst. Specifically, this reference discloses introducing a heavy aromatic feed along with pure toluene during toluene disproportionation while maintaining toluene conversion levels and without adversely affecting catalyst activity and aging quality. While Shamshoum '180 discloses the conversion of heavy aromatic compounds, the amount of heavy aromatics that can be processed is limited. Because Shamshoum '180 is concerned with the constant conversion of toluene, toluene must make up the majority of the feed. Shamshoum '180 does not disclose processing feed streams composed of large amounts of heavy aromatics, or feedstreams that are made up primarily or entirely of such heavy aromatics. It would therefore be beneficial to provide a method of converting heavy aromatic compounds in large quantities and without adversely affecting catalyst activity or reducing catalyst life for use in toluene disproportionation.